With reference to FIG. 1, an electro-dynamic loudspeaker having a conventional design includes:                a permanent magnet assembly typically comprising metal components 1a and a permanent magnet 1b         a voice coil assembly typically comprising a wire referred to as a voice coil 2a wound/wrapped around a thin tube referred to as a voice coil former 2b         a diaphragm 3        a chassis 5 to which other acoustic components are secured        a suspension system which suspends the diaphragm 3 from the chassis 5, typically including an edge suspension 4a and a spider 4b         
This design is widely accepted in the industry because of the mechanical stability it provides.
The electro-dynamic loudspeaker of FIG. 1 works on the same basic principle as most electro-dynamic loudspeakers. A wire with a certain length is wound around the voice coil former 2b to form a solenoid more commonly referred to as the voice coil 2a. This forms the voice coil assembly which is positioned in an air gap 6. A static magnetic field is generated by the permanent magnet 1b in the air gap 6 and hence the voice coil assembly is positioned inside the static magnetic field generated by the permanent magnet 1b. When a current is applied at terminals of the solenoid, the solenoid generates a magnetic field which counteracts with the static magnetic field generated by the permanent magnet 1b of the permanent magnet assembly. The permanent magnet assembly and voice coil assembly collectively form a drive unit, with the permanent magnet assembly providing a stationary part of the drive unit secured to the chassis 5 and the voice coil assembly providing a translatable part of the drive unit secured to the diaphragm 3.
Since the permanent magnet assembly is secured to a rigid surface of the chassis 5, the force which is generated by the flowing current moves the diaphragm 3. The motion of the diaphragm 3 is preferably limited to a predetermined axis 8, which may be an axis of symmetry of the diaphragm 3, by the suspension system, preferably so that the voice coil 2 will always work in the most efficient portion of the static magnetic field generated by the permanent magnet 1b and will not come into contact the metal components 1a of the permanent magnet assembly (in the air gap 6).
A critical part of manufacturing and designing an electro-dynamic loudspeaker is to define the air gap 6. To improve the efficiency of a loudspeaker, it is desirable to design this air gap 6 to be as small as possible. Higher magnetic flux densities in the air gap 6 results in higher forces generated by the same current passing through the voice coil 2a windings compared to lower magnetic flux density in the air gap 6. Therefore the narrower the air gap 6 is, the higher the potential efficiency of the loudspeaker.
However, there are other design aspects which may force a designer to consider a wider air gap 6. One of them is the nature of the suspension system. The suspension system is typically made of soft materials, such as rubber, textile or paper, to enable the movement of the diaphragm 3 along the predetermined axis 8. Any movement of the diaphragm 3 and the voice coil assembly, which is mounted to it, in any direction other than along the predetermined axis 8 brings the voice coil 2a and the voice coil former 2b closer to the metal components 1a of the permanent magnet assembly. A contact, in the worst case, may generate unwanted noise or breaks contacts in the voice coil 2a, thereby stopping operation of the loudspeaker.
The suspension system preferably has two functions in an electro-dynamic loudspeaker. It preferably functions as a part of a single degree of freedom system ensuring that the diaphragm 3 moves only along the predetermined axis 8, and preferably further provides any required damping and/or stiffness to be able to fulfill a set of performance requirements. Thus, the suspension system preferably mechanically centres the voice coil assembly 2 in the air gap 6 during the travel (translation) of the diaphragm along the predetermined axis 8. For the suspension system 4 to be able to fulfill these preferred requirements, the components have to be designed within certain rules.
A suspension system usually includes two components, the first is referred to herein as the edge suspension 4a and the second is referred to herein as the spider 4b. The spider 4b may have other names in the industry, e.g. the “centering ring”. The name “centering ring” reveals more about the function the spider 4b delivers in an electro-dynamic loudspeaker. The edge suspension 4a contributes to the stiffness and the damping parameters and also preferably establishes a sealing function between the front and the back sides of the diaphragm 3 thereby contributing to pressure built-up for the generation of sound.
The edge suspension 4a is typically mounted around the diaphragm 3 with a contact surface at an inner edge of the edge suspension 4a being attached to a contact surface at an outer edge of the diaphragm 3. A contact surface at an outer edge of the edge suspension 4a is preferably attached to a contact surface on the chassis 5 of the loudspeaker. The edge suspension 4a may e.g. be made out of rubber, foam, textile or paper in order to enable diaphragm 3 to move along the predetermined axis 8 of the loudspeaker. A contact surface at an inner edge of the spider 4b preferably attached to a contact surface on the voice coil former 2b. A contact surface at an outer edge of the spider 4b is preferably attached to a contact surface on the chassis 5.
The distance between (i) the contact surface on the chassis 5 at which the edge suspension 4a is attached to the chassis 5; and (ii) the contact surface on the chassis 5 at which the spider 4b is attached to the chassis 5 is referred to herein for brevity as the “distance between the suspension elements”. This distance is directly related to the stability of the voice coil assembly 2 movement within the air gap 6.
The distance between the suspension elements can be changed depending on the application(s) intended for the loudspeaker. This distance is related to the height of a given loudspeaker and is further dependent on the dimensions of the diaphragm 3, how much excursion of the diaphragm 3 is required, the dimensions of the voice coil 2a as well as other parameters which affect the performance of the loudspeaker. In some cases, the distance between the suspension elements may need to be very small for a given loudspeaker, and may even be insufficient to establish the basic requirements of stable operation, for example when the electro-dynamic loudspeaker is needed to be built as flat as possible. Under these conditions, there may not be enough room for a spider 4b, but removal of the spider 4b can make a loudspeaker unstable to rocking. It is difficult to make a loudspeaker having the required stability with an edge suspension 4a being the only suspension element (i.e. with the spider 4b omitted).
It is common that, when there is no room for two suspension elements in a loudspeaker, the spider 4b is omitted. This is due to the fact that the edge suspension 4a normally functions as a sealing element between the front and back sides of the diaphragm 3 and therefore contributes to pressure built-up. When the spider 4b is omitted from the loudspeaker, the stability of the voice coil assembly in the air gap is greatly reduced. In this case, a resemblance can be made between a pendulum and the loudspeaker without a spider 4b. A heavy voice coil assembly, hung at the bottom of a diaphragm 3, is free to move in directions other than the predetermined axis 8.
Many attempts have been made to reduce the total height of a loudspeaker. One example is U.S. Pat. No. 6,385,327 B1, see in particular FIGS. 1, 2 and 3 of this document. In this document, a first set of blade springs which have a triangular cross-section are used around the diaphragm (providing an edge suspension) with a second set of blade springs being used around the voice coil former (providing a spider) to eliminate the movement of the voice coil assembly in the horizontal plane, making it possible for the diaphragm to move in along a symmetry axis of the loudspeaker. However, the blade springs are arranged in such a way that there are gaps (openings) in between adjacent blade springs, and so the blade springs fail to establish a sealing functionality, unlike like a more conventional edge suspension 4a as described above with reference to FIG. 1.
As shown by FIGS. 1, 2 and 3 of U.S. Pat. No. 6,385,327 B1, in the triangular cross-section of the blade springs, the bottom side of the triangle is left out so that two sides of the triangle are formed from a material. This enables the diaphragm to move freely over the symmetry axis of the loudspeaker.
As shown by FIG. 1 of U.S. Pat. No. 6,385,327 B1, the blade springs of U.S. Pat. No. 6,385,327 B1 leave openings in corner regions that are between the blade springs. These openings do not contribute to the pressure built-up (sealing between the front and back sides of the diaphragm) by the edge suspension of U.S. Pat. No. 6,385,327 B1. The absence of edge sealing is generally not desirable since it introduces acoustic short-cuts and greatly reduce the performance of a loudspeaker.
U.S. Pat. No. 4,056,697 introduces static or rigid elements which are closely positioned between the springs, with a view to improving the sealing of blade spring systems. This application also discloses a technique which allows the blade springs to be used without a spider element (unlike U.S. Pat. No. 6,385,327 B1, discussed above). As shown in FIG. 6 of U.S. Pat. No. 4,056,697, static or rigid elements 37 are introduced to reduce the size of the openings between the blade springs as much as possible to prevent the communication of the front side of the diaphragm with the back (or “rear”) side and reduce the acoustic short cuts by sealing the edge of the diaphragm. However, whilst the elements 37 shown in FIG. 6 of U.S. Pat. No. 4,056,697 reduce the size of the openings between the blade springs, it is evident that this reduction does not provide complete sealing, and so when the loudspeaker of U.S. Pat. No. 4,056,697 is mounted in a small enclosure and has to make large volume displacements, the pressure difference between the front and the back sides of the diaphragm can becomes high enough for these small openings to introduce what are known as blowing noises. This blowing noise is a distinct problem in the industry and happens when there is a high velocity flow between a pressure maximum and a pressure minimum. Blowing noises are not desirable in loudspeaker designs and attempts are usually made to avoid this where possible.
For a shallow loudspeakers including a single suspension element, stability is very important. It is desirable for the loudspeaker to be as stable as possible, meaning that the voice coil should only move along a predetermined axis, with minimal off-axis (e.g. horizontal) displacement. However, if the loudspeaker has only a single suspension element, it will generally be unstable due to the introduction of rocking modes, which means that the voice coil will have some horizontal movement. To allow for these rocking modes, a designer will have to increase the size of the air gap in the loudspeaker design to prevent the voice coil assembly contacting the magnet assembly, reducing the efficiency of the design. This links the efficiency of the loudspeaker with the stability provided by the geometry of the edge suspension.
There have been proposals to reduce the displacement of the voice coil assembly in the horizontal plane. Blade springs and hinge mechanisms for blade springs have been proposed. However, known blade spring designs have air leakage and sealing problems between the front and the back sides of the diaphragm, as discussed above. The leakage problem introduces undesired effects that can be worse than those introduced by having a wide air gap. The present inventors believe this suggests that the edge suspension should where possible provide a seal between the front and the back sides of the diaphragm.
The present invention has been devised in light of the above considerations.
The present inventors believe it would be desirable to find a geometry for an edge suspension capable of being used in as the only suspension element in a loudspeaker that would eliminate the rocking of the voice coil assembly in the air gap, without degrading the linearity of the suspension, while also establishing a good seal between the front and the back sides of the diaphragm.